The structures of several related heart muscle enzymes are being determined using the methods of single crystal x-ray diffraction. The determination of the molecular structure of cytoplasmic malate dehydrogenase (s-MDH) has reached the final stages. Only the side chain electron density has not been fully interpreted in terms of a molecular model. The position in the enzyme and the conformation of bound coenzyme, NAD ion, is now known in detail. Small deviations from two-fold rotational symmetry are found in the subunits of the dimeric s-MDH particularly in the active site region. These conformational differences will be documented and an attempt will be made to correlate them with the catalytic properties of the enzyme. A similar structural analysis has been started on Beta-hydroxy acyl coenzyme dehydrogenase Beta HADH) and the preparation of crystals of mitochondrial malate dehydrogenase (m-MDH) is in progress. Both enzymes are mitochondrial enzymes and the long-range goal is to describe the parts of the molecular structures which are characteristically different from related cytoplasmic enzymes. The studies on Beta HADH will first be focussed on extending the radiation lifetime of the crystals.